The only way to hide an app on an iPhone is to change the age limits in Restrictions. Or, by jailbreaking the phone.
Did this person have access to the phone for some extended period of time while it was unlocked? If so, restore the phone from a backup made before the intrusion. If you don't have a trustworthy back up, restore it as new.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Going forward, keep a passcode on your phone and keep it locked.